1. Field of the Invention
This invention relates to a vacuum chamber and, more particularly, to a circuit and apparatus for verifying evacuation of the chamber after a substrate is loaded into the chamber for processing.
2. Description of the Related Art
A substrate is generally defined as a material upon which processing takes place. For example, a substrate can receive deposition, etch, implant, heat (anneal) and various clean cycles. A popular type of substrate is one which is silicon-based and generally known as a semiconductor wafer. Substrate, however, is a term henceforth used to encompass any object which receives processing and therefore is defined to extend beyond a semiconductor wafer.
In many instances, optimal substrate processing involves placing the substrate inside a chamber containing an ambient dissimilar from that which surrounds the chamber. Ambient within the chamber may be controlled to ensure processing is carried out according to a specific processing "recipe". Using the semiconductor wafer example, certain types of deposition require wafers within a chamber be subjected to a vacuum and substantial temperatures. Low pressure, high temperature deposition upon the wafer are well recognized as, for example, low pressure chemical vapor deposition ("LPCVD"). Low pressure processing to produce a LPCVD film is but one example of many involving a chamber and pressurization within that chamber. Pressurization levels within a chamber to effectuate optimal processing of the substrate is critical not only to produce accurate film properties, but also to minimize contamination within the chamber during any or all of the various processing steps beyond deposition.
When film is being deposited, etchant delivered, dopants implanted or temperatures applied, it is important that extraneous material not be allowed to enter the chamber during those critical processing steps. For example, if gas or particulate matter enters the chamber during LPCVD processing, the ensuing film may suffer improper conformality and/or electrical properties. It is therefore desired that the chamber ambient receive proper pressurization levels before processing is applied to the substrate. This implies that all apertures and/or orifices into the chamber be sealed after the substrate is loaded and before the substrate is processed.
FIG. 1 illustrates one example of an application involving a chamber 10 which can be pressurized (i.e., a pressurized chamber). A mechanism 12 is used for loading one or more substrates 14 into chamber 10 prior to evacuating the chamber. In the example provided, chamber 10 may be configured as a horizontal tube LPCVD reactor which can be used to deposit doped or undoped dielectric films upon exposed surfaces of substrate 14. Further to that example, substrate 14 may be a silicon-based semiconductor wafer fixed on edge within a carrier secured to a paddle or plunger 16. Thus, the carrier which secures one or more wafers, defined as substrate 14, may be secured to a door 18 to which plunger 16 is coupled. Within the semiconductor fabrication industry, chamber 10 can be referred to as a furnace or reactor, which can receive radiated heat via resistive heating coils surrounding the reactor. Chamber 10 interior walls may be made of quartz or metallic material which are somewhat inert to reactant gasses 20 metered into chamber 10 at one end of the chamber. The other end of chamber 10 is coupled to a vacuum pump which extracts reactant byproducts 22 in the direction shown.
Metering the reactant gasses 20, and evacuating byproducts 22 occur only if it is known that the inside of chamber 10 is properly sealed. This requires knowing that door 18 is properly sealed against exterior ambient ingress. Door 18 may be designed to close in the direction of arrows 24 until a seal occurs between a flange 26 at one end of chamber 10 and a sealing membrane 28 arranged on a chamber-facing surface of door 18. Membrane 28 may comprise an o-ring, and flange 26 may be stainless steel or quartz. Flange 26 comprises a relatively planar surface upon which membrane 28 can be compressed when door 18 closes. Once compressed, membrane 28 seals the interface between door 18 and chamber 10.
Before processing can occur (i.e., before reactant gas is metered into the chamber and byproducts removed), it is important to ensure the chamber's integrity by verifying closure of door 18. Many conventional mechanisms are available to provide door closure verification. A popular mechanism may involve a proximity switch 30 mounted to the exterior surface of chamber 10. Proximity switch 30 is generally considered a "contact" switch which movably reciprocates when placed in contact with a protrusion 32 of door 18. Merely as an example, door 18 may include a protrusion 32 and chamber 10 exterior housing may include a receptacle 33. As protrusion 32 enters receptacle 33 toward which switch 30 extends, protrusion 32 surface will contact and thereafter move the switch lever from a normally open position to a closed position. Of course, there may be numerous other mechanisms on which switch 30 is mounted and from which switch 30 is activated. The receptacle/protrusion mechanism is shown only as an example.
Entry of protrusion 32 into the receptacle 33 of chamber 10 hopefully coincides with the compression (i.e., seal) of membrane 28 against flange 26. When moved, protrusion 32 should manually brush against the lever of switch 30, causing that lever to transition upon a contact as shown in further detail in FIG. 2. FIG. 2 illustrates closure of lever 34 between a pair of contacts 36a and 36b. Closure of lever 34 across contacts 36 produces a signal from switch 30 upon conductor 38.
In many instances, there may be two switches. A second switch 40 may be present as shown in FIGS. 1 and 2. Switch 40 may suffice as a back up if switch 30 is rendered inoperable. Switch 40 may also serve as a fail stop to prevent protrusion 32 from unduly pressing against chamber 10 housing should switch 30 fail. Switch 40, similar to switch 30, comprises a pair of contacts 42a and 42b, and a lever 44 which reciprocates between contacts 42. A conductor 46 extends from switch 40 to indicate status of the switch and, more importantly, whether door 18 is closed against flange 26.
The nature of proximity switches in general and the mechanism by which they are mounted does not always allow accurate verification of door closure. Proximity switches rely upon movement of the levers 34 and 44 when contacted with protrusion 32. If the protrusion and/or door should become misaligned, then actuation of levers may not occur indicating the door is open when in fact it is closed. Furthermore, integrity of the contacts may be lessened through extensive wear, also resulting in a switch that is open when the door is actually closed. Conversely, membrane 28, and the integrity by which membrane 28 mates with flange 26, may be reduced after repeated door closure so that the seal between those structures will eventually fail even though switches 30 and 40 indicate switch closure.
Conventional proximity switches and their use in verifying door closure is therefore not always absolute. In actuality, proximity switches mostly determine the status of the switches and not necessarily the status of door closure or, more importantly, status of pressure within the chamber. If the door is closed but the switches fail and do not indicate closure, then the processing tool will hold off its processing until the misreading problem has been solved. This can reduce processing throughput. Conversely, if the switches indicate the door is closed and yet the door is open, then gasses external to the chamber will be drawn into the chamber along with the evacuated byproducts. Ingress of, for example, oxygen or hydrogen into chamber 10 during LPCVD oxide or nitride formation may contaminate the ensuing dielectric film and jeopardize the electrical properties of that film. In a worst case situation, toxic or flammable gasses may enter the chamber through the partially open door 18, causing a possible fire or a toxic cloud to form within the chamber. A more likely scenario is the door is slightly ajar even though the switches indicate the door is sealed against chamber 10. When vacuum is drawn to the chamber, particles are agitated across the substrate as turbulence is created through the slight opening between the chamber 10 and door 18. Sufficient particles within what should be a pristine deposited film may cause significant yield loss of the final product. In the case of semiconductor wafers, particulate count on the silicon substrate or within a deposited oxide film may, in the extreme, cause improper shorting of dielectrically spaced conductors.
It is therefore desired that a mechanism be derived which can accurately verify door closure in a pressurized chamber. Instead of monitoring switch operation, it would be more prudent to monitor pressure levels within the chamber since, in fact, it is pressure level that dictates whether or not the door is actually closed and sealed.